1. Field of the Invention
The invention relates to digital communications systems and, more particularly, to a receiver architecture that is characterized by a relatively low intermediate frequency (IF) section, followed by polyphase filter that results in enhanced image rejection, and a digital I/Q demodulator.
2. Description of the Related Art
Digital modulation and demodulation techniques incorporating I/Q (In-phase/Quadrature) modulators and demodulators are widely used in communication systems. I/Q demodulators are abundantly discussed in the technical literature. See, for example, Behzad Razavi, RF Microelectronics, Prentice Hall (1998) and John G. Proakis, Digital Communications, McGraw-Hill (1995). There exists also patent art related to the technology of I/Q modulation and demodulation: U.S. Pat. No. 5,974,306, entitled xe2x80x9cTime-Share I/Q Mixer System With Distribution Switch Feeding In-Phase and Quadrature Polarity Invertersxe2x80x9d to Homak, et al.; U.S. Pat. No. 5,469,126, entitled xe2x80x9cI/Q Modulator and I/Q Demodulatorxe2x80x9d to Murtojarvi.
Examples of digital communications system applications that incorporate and standardize I/Q modulation and demodulation include the GSM (Global System for Mobile Communications), IS-136 (TDMA), IS-95 (CDMA), and IEEE 802.11 (wireless LAN). I/Q modulation and demodulation have also been proposed for use in Bluetooth wireless communication systems.
Bluetooth devices are capable of interlinking to form piconets, each of which may have up to 256 units, with one master and seven slaves active while others idle in standby nodes. Piconets can overlap, and slaves can be shared. In addition, a form of scatternet may be established with piconets overlapping, thereby allowing data to migrate across the networks.
The Bluetooth system operates in the 2.4 GHz ISM (Industrial, Scientific, Medical) band, and devices equipped with Bluetooth technology are expected to be capable of exchanging data at speeds up to 720 Kbs at ranges up to 10 meters. This performance is achieved using a transmission power of 1 mw and the adoption of frequency hopping protocols to avoid interference. In the event that a Bluetooth-compatible receiving device detects a transmitting device within 10 meters, the receiving device will automatically modify its transmitting power to accommodate the range. The receiving device is also required to operate in a low-power mode as traffic volume abates, or ceases altogether.
Bluetooth devices are capable of interlinking to form piconets, each of which may have up to 256 units, with one master and seven slaves active while others idle in standby nodes. Piconets can overlap, and slaves can be shared. In addition, a form of scatternet may be established with piconets overlapping, thereby allowing date to migrate across the networks.
The invention addressed herein is driven by the long-standing requirement, applicable with equal force to Bluetooth designs, to eliminate, or least minimize, the need for external filters commonly encountered in the design of contemporary double-conversion communications receivers. An example of the typical double-conversion receiver architecture is illustrated in FIG. 1. That architecture requires a first bandpass RF filter 21 disposed between antenna 10 and RF amplifier stage 20. A second bandpass RF filter couples the output of RF amplifier 20 to a first input of mixer 30. The primary function of RF bandpass filters 21 and 22 is to effect front-end selectivity, thereby enhancing the receiver""s image response performance, as well as affording protection against spurious responses related to, for example, intermodulation and cross-modulation phenomena. However, because the selectivity provided by filters 21 and 22, in general, varies inversely with the insertion loss thereby caused, the level of selectivity attainable is limited by system design constraints. Furthermore, RF bandpass filters are not conveniently realizable in integrated circuit form. Consequently, the necessity of coupling outboard RF filters at strategic points to otherwise integrated receiver circuitry increases the manufacturing complexity and cost, as well as the physical size of the receiver.
With continued reference to the receiver architecture depicted in FIG. 1, the RF carrier is first converted to IF in mixer 30. The LO signal to mixer 30 is synthesized from a phase-locked oscillator that includes a VCO 50 and phase-locked loop (PLL) 40. The output of mixer 30 is coupled to an IF bandpass filter 31. The paramount functions of the IF bandpass filter are to establish channel selectivity and to define the noise bandwidth of the receiver. The output of IF bandpass filter 31 is coupled to the input of amplifier 60. The output of amplifier 60 is coupled to one input of demodulator 70. The second input to modulator 70 is derived by processing the output of amplifier 60 through quad tank 61. The output of demodulator 70 is filtered by low-pass filter 71, and NRZ data is recovered in a bit slicer 80 that operates synchronously with the SCLK signal.
What is notable with respect to the above receiver architecture, and underscored in FIG. 1, is the necessary inclusion of no fewer than four outboard filters, BPFs 21, 22 and 31, and quad tank 61. These filter elements are not readily realizable with resort to contemporary integrated circuit technology. Bandpass RF filters 21 and 22 are frequently implemented with surface acoustic wave (SAW) devices, and the IF bandpass filter 31 often requires a crystal filter. Quad tank 61 may be predictably constructed from lumped passive circuit elements. It is readily appreciated that the necessary inclusion of these filter elements frustrate, or at least compromise, the objective of achieving a small, compact and easily transportable communications receiver. Accordingly, what is desired is a receiver architecture that satisfies system requirements such as selectivity, image rejection and noise figure, while limiting the dependence on non-integrable frequency-selective components.
The above and other objects, advantages and capabilities are achieved in one aspect of the invention by a communications receiver that comprises an amplifier for coupling to an input carrier signal; an I demodulator, coupled to the output of the amplifier; a Q demodulator, coupled to the output of the amplifier; a quadrature LO generator for coupling to an LO signal source, the quadrature LO generator providing an LO_I output to the I demodulator and providing an LO_Q output to the Q demodulator; a polyphase filter having a first input coupled to the output of the I demodulator and having a second input coupled to the output of the Q demodulator; a first A/D converter having an input coupled to a first output of the polyphase filter; a second A/D converter having an input coupled to a ""second output of the polyphase filter; and a digital I/Q demodulator having first and second inputs respectively coupled to the first and second outputs of the polyphase filter. In greater detail, a preferred embodiment of the communications receiver is designed to have a low IF, approximately 1 MHz; and the polyphase filter is constructed to have a first input node coupled to the output of the I demodulator; a second input node coupled to the output of the Q demodulator; a first output node coupled to the input of the first A/D converter; a second output node coupled to the input of the second A/D converter; a reference node; a plurality of interior nodes; a plurality of capacitive elements; and a plurality of gyrator elements, each having an associated input terminal and an associated output terminal; and wherein: (i) a respective capacitive element is coupled between each input node and the reference node, between each output node and the reference node, and between each interior node and the reference node, and (ii) a gyrator element is coupled between the input nodes, between the output nodes, between each input node and one internal node, between each output node and one internal node.
In another aspect, the invention is intended to realize a method of digital demodulation, preferably effected through a single integrated-circuit device, and with minimal reliance on outboard frequency-selective elements, such as SAWs, crystal filters, and the like. The method comprises the steps: applying a carrier signal, at a frequency RF, to the respective signal inputs of an I demodulator and a Q demodulator; applying the outputs of a quadrature LO generator to the respective LO inputs of the I demodulator and the Q demodulator; coupling the output of the I demodulator to the I input of a polyphase filter; coupling the output of the Q demodulator to the Q input of a polyphase filter; performing an A/D conversion of the I output of the polyphase filter; performing an A/D conversion of the Q output of the polyphase filter; coupling the A/D-converted I and Q outputs of the polyphase filter to the inputs of a digital I/Q demodulator; and recovering data through the operation of the I/Q demodulator. More specifically, the method contemplates conversion of the RF carrier to a relatively low IF, approximately +1 MHz. In addition, the method, in a particularized embodiment, is predicated on the operation of a polyphase filter constructed to have a, first input node coupled to the output of the I demodulator; a second input node coupled to the output of the Q demodulator; a first output node coupled to the input of the first A/D converter; a second output node coupled to the input of the second A/D converter; a reference node; a plurality of interior nodes; a plurality of capacitive elements; and a plurality of gyrator elements, each having an associated input terminal and an associated output terminal; wherein: (i) a respective capacitive element is coupled between each input node and the reference node, between each output node and the reference node, and between each interior node and the reference node, and (ii) a gyrator element is coupled between the input nodes, between the output nodes, between each input node and one internal node, between each output node and one internal node.
In further ramification, the invention resides in a mixer/demodulator for a communications receiver. The mixer/demodulator comprises an input terminal for a carrier signal at a carrier frequency; a local oscillator (LO) signal source; a quadrature LO generator coupled to the local oscillator signal source for providing an in-phase LO signal LQ_I, and a quadrature LO signal, LO_Q; an I mixer (13) having a carrier input coupled to the input terminal and an LO_I input coupled to the LO_I signal from the quadrature LO generator; a Q mixer (14) having a carrier input coupled to the input terminal and an LO_Q input coupled to the LO_Q signal from the quadrature LO generator; wherein: (i) the frequency of the LO_I signal is equal to the frequency of the LO_Q signal and that frequency is only slightly offset from the carrier frequency, and (ii) the LO_I signal and LO_Q signal have a predetermined phase relationship; filter means coupled to the output of the I mixer and to the output of the Q mixer for substantially reducing the response of the communications receiver to an image signal; and a digital I/Q demodulator coupled to the output of the filter means. In a particularized embodiment, the polyphase filter is constructed to have a first input node coupled to the output of the I demodulator; a second input node coupled to the output of the Q demodulator; a first output node coupled to the input of the first A/D converter; a second output node coupled to the input of the second A/D converter; a reference node; a plurality of interior nodes; a plurality of capacitive elements; and a plurality of gyrator elements, each having an associated input terminal and an associated output terminal; and wherein: (i) a respective capacitive element is coupled between each input node and the reference node, between each output node and the reference node, and between each interior node and the reference node, and (ii) a gyrator element is coupled between the input nodes, between the output nodes, between each input node and one internal node, and between each output node and one internal node.